As the demand for diesel fuel and aviation fuel increases worldwide, there is increasing interest in sources other than petroleum crude oil for producing the fuels. One such source is what has been termed renewable sources. These renewable sources include, but are not limited to, plant oils such as corn, rapeseed, canola, soybean and algal oils, animal fats such as inedible tallow, fish oils, and various waste streams such as yellow and brown greases and sewage sludge. The common feature of these sources is that they are composed of glycerides and Free Fatty Acids (FFA). Both of these compounds contain aliphatic carbon chains having from about 8 to about 24 carbon atoms. The aliphatic carbon chains in the glycerides or FFAs can be saturated or mono-, di- or poly-unsaturated. The glycerides may be tri-glycerides, di-glycerides, mono-glycerides, or any combination thereof.
There are reports in the art disclosing the production of hydrocarbons from oils. For example, U.S. Pat. No. 4,300,009 discloses the use of crystalline aluminosilicate zeolites to convert plant oils such as corn oil to hydrocarbons such as gasoline and chemicals such as para-xylene. U.S. Pat. No. 4,992,605 discloses the production of hydrocarbon products in the diesel boiling range by hydroprocessing vegetable oils such as canola or sunflower oil. Finally, US Publication No. 2004/0230085 discloses a process for treating a hydrocarbon component of biological origin by hydrodeoxygenation followed by isomerization.
Processes for producing two fuels, such as a diesel fuel and an aviation fuel, from renewable feedstocks are also known. The aviation fuel is produced via operation of the isomerization/cracking reactor in a higher severity mode to induce greater isomerization and cracking on longer chain n-paraffins (typically nC15-nC18).
FIG. 1 illustrates one example of such a process 100. The renewable feed 105 is sent to a hydrogenation and deoxygenation zone 110 where hydrogen 115 is added. The reaction mixture includes a liquid portion and a gaseous portion. The gaseous portion 120 comprises unreacted hydrogen, carbon dioxide, carbon monoxide, water vapor, propane and possibly sulfur, phosphorous, or nitrogen components. The liquid portion includes a hydrocarbon stream 130 and a liquid water stream 125.
The hydrocarbon stream 130, which contains n-paraffins, is sent to an isomerization and selective hydrocracking zone 135 where hydrogen 140 is added. Some n-paraffins are isomerized to branched paraffins, and some longer chain n-paraffins are hydrocracked to shorter chain paraffins.
The effluent 145 from the isomerization and selective hydrocracking zone 135 is sent to a fractionation zone 150 where it is separated into various streams including a light ends stream 155, a naphtha stream 160, an aviation fuel stream 165, and a diesel stream 170. A portion 175 of the diesel stream 170 can be recycled to improve the aviation fuel yield.
Typical isomerization and selective cracking conditions suitable for producing a large amount of aviation fuel boiling-range product for the flow scheme depicted in FIG. 1 include a temperature of about 165° C. to about 375° C. and a pressure of about 1724 kPa absolute (250 psia) to about 4826 kPa absolute (700 psia). In another embodiment, the isomerization conditions include a temperature of about 295° C. to about 375° C. and a pressure of about 3102 kPa absolute (450 psia) to about 3792 kPa absolute (550 psia). Other operating conditions for the isomerization zone are well known in the art.
Unfortunately, one side effect of making significant amounts of aviation fuel is that co-product naphtha and light ends are also produced in higher quantities at the expense of distillate yield. These naphtha and light ends co-products are undesirable because they have much lower value than the potential distillate products that could be otherwise produced.
Therefore, there is a need for a process of producing aviation fuel and diesel at high conversion rates from renewable feedstocks with lower quantities of naphtha and lights ends.